Cheating is a term that describes the use of third-party programs or material changes in order to gain an unfair advantage in gameplay. Valve has a strict policy against cheating and will ban detected cheats with the Valve Anti-Cheat system (abbreviated to "VAC").
Aimbots automatically aim at an enemy for the player. This can be customized to ignore Spy's cloak, disguise, or Medic's Vaccinator and have "legit" features (features which attempt to disguise cheating behavior) such as smoothing.
Anti-aim is to constantly change the direction you are looking to prevent being headshot. This is sometimes done by modifying the view angles of the player by editing the values in pitch or yaw into an out-of-bound value, so the player can appear facing one direction, when in reality the real hitbox is facing the other direction. This is often referred to as a "fake angle".
Anti-anti-aim, or Aim resolver, attempts to brute-force the enemy's real model by shooting at certain locations until a headshot occurs. Certain cheats can also "force" an enemy cheater's viewangles, so that they may manually resolve their anti-aim.
Autostrafing, or auto airstrafing, refers to automatically airstrafing left or right when your mouse moves left or right. Also has a silent variant, as well as a directional variant which allows pressing movement keys to move in directions that the cheater is not facing.
If a player or bot is holding the Intelligence or Mann vs. Machine bomb and they get backtracked, the Intelligence (or bomb) gets teleported to the position they were backtracked to. NPCs, such as the Tank Robot or ghosts, can not be backtracked due to them not being lag compensated. Cheaters cannot backtrack with projectile weapons due to the same reason.
It's possible to "store" random Crits by filling up the "crit bucket", which can then be used at will by a cheater. Melee weapons do not follow this Crit bucket system, which allows them to Crit 100% of the time with cheats.
Fake latency refers to artifically increasing or decreasing ping to the server. Decreasing ping has no effect on gameplay and the real ping can be found by typing ping in the console. Increasing ping will also increase the amount of lag compensation done by the Source Engine, allowing cheaters to shoot people behind walls much easier.
This introduces a big imprecision (as it gets rounded more and more). The client can predict the pattern generated by the server as you only need a value approximately close to that of the servers, and the rounding will do the rest, which will allow for no spread-like properties to work. Casual servers generally do not stay up long enough for this cheat to be effective, but some community servers may.
Originally a bug that would allow player actions when taunting if you taunted right before you fell off something. Cheaters took advantage of this so they could taunt slide whenever they taunted, without needing to fall off anything.
Another form of speedhacking rose soon after the original speedhacking was patched out, known as "viewangle speed hack"; it would allow for higher than normal velocities, but wouldn't have the same effects of normal speedhacking.
Since late 2019, bots (semi/fully automated cheating programs with little to no input from humans) have been created to join Casual servers using aimbot cheats, server lag, spinbot, walkbot, and votekicking real players, among other things.
Several updates have attempted to fix or mitigate the voice and text chat spam, name stealing, votekick initiation, team name changing, server lagging, and the unequipping of cosmetics. As well as several VAC ban waves to cheating bot accounts, and game bans on accounts involved in botting activities.
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